1. Field of Invention
The present invention relates generally to a propulsion system containing a mixed-phase oxidizer and/or a mixed phase fuel propellant, and to a method for propelling an object containing such propulsion system. The propulsion system and method of the present invention are particularly adaptable for use with rockets.
2. Description of Related Art
Bipropellant propulsion systems are well known in the art and generally contain an oxidizer stored separately from a reactive fuel propellant. In monopropellant propulsion systems, the oxidizer and fuel propellants are stored in combination.
Hybrid propellant systems, a species of bipropellant propulsion systems, have been gaining more acceptance and attention. A hybrid propellant system typically contains of a fluid oxidizer retained in an oxidizer storage chamber, a solid fuel grain such as a molded cylinder of hydroxyl-terminated polybutadiene rubber or polyethylene polymer retained in a pressure vessel, a mixing chamber, an injector for introducing the fluid oxidizer into the solid fuel grain, and a nozzle which in operation discharges exhaust gases so as to provide forward thrust to the system. Some of the more well-known advantages associated with hybrid propulsion systems include the complete separability of the fuel from the principal oxidizer, thus inhibiting the potential for inadvertent ignition or catastrophic failure; the flexibility in selecting and optimizing the combination of propellant ingredients regardless of whether they are solid or liquid; and the ability to easily start, stop, and restart the propulsion system. Reverse hybrid propellant systems, which typically contain a liquid fuel propellant retained in a storage chamber and a solid oxidizer retained in a separate pressure vessel chamber, theoretically provide similar advantages to a hybrid system. To date, however, reverse hybrid rocket motors have not been well developed, owing to difficulties associated with development of a solid oxidizer structure possessing satisfactory thermo-mechanical properties, such as strength, durability, and ablation-rate control.
In addition to the above-mentioned safety and flexibility concerns, in designing a propulsion system attention must also be given to the performability and efficiency of the oxidizer and fuel propellants employed in the system. In particular, the capability of the oxidizer and fuel propellants to enhance the following two measurable properties is especially pertinent in the selection of the propellants:
(1) the specific impulse of the system, which is defined as the thrust in pounds force developed by a specific system, multiplied by the duration in seconds of the thrust, divided by the weight in pounds of the fuel and oxidizer; and
(2) the density specific impulse of the system, which is defined as the specific impulse multiplied by the specific gravity of the fuel-oxidizer combination.
One manner of optimizing these properties in a given propulsion system is to increase the density of the oxidizer and fuel propellants. By increasing the density of these propellants, a corresponding decrease in the size of the oxidizer and fuel tanks that must be propelled through the atmosphere is realized. Consequently, the attendant aerodynamic drag of the propulsion system is abated.
Conventional liquid-phase oxidizers usually are constituted by a single chemical species, such as, by way of example, liquid oxygen, liquified nitrous oxide, red fuming nitric acid, nitrogen tetroxide, liquified fluorine, or hydrogen peroxide. In addition, gases, such as gaseous oxygen or fluorine, and mixtures of liquid and gas oxidizers, such as fluorine-liquid oxygen, have occasionally been employed. However, each of these liquid and liquid/gas oxidizers have been plagued by such deficiencies as: poor specific impulse and density specific impulse; a short storageability; high toxicity, especially in the cases of nitric acid, nitrogen tetroxide, and fluorine; instability, especially in the case of hydrogen peroxide; and any combination thereof.
In an attempt to develop a higher performance oxidizer suitable for introduction into a combustion chamber by fluid flow, the present inventors envisioned combining solid-phase oxidizers and additives with liquid oxidizers. However, fluid-solid (i.e., mixed-phase) oxidizers initially were considered unsuitable for a propulsion system, given the tendency of the propellants of the mixture to separate during storage, resulting in a stratified liquid-solid, gas-solid, or gas-liquid-solid charge in the oxidizer storage chamber.
In order to prevent the solid and fluid phases from separating in storage, it was proposed to divide the solid-phase propellant into sufficiently fine particulates which would behave in a colloidal manner when mixed with the fluid phase. Colloidal mixtures, sometimes referred to as emulsions or gels, have been known to maintain their suspension of solid particles indefinitely. However, the provision of colloidal mixtures in the propulsion system would be accompanied by several adverse consequences. For example, some oxidizer materials, such as ammonium nitrate and ammonium perchlorate, would be difficult to reduce to colloidal size due to their hygroscopic nature. Also, highly-energetic oxidizers such as nitronium perchlorate, ammonium dinitramide, glycidyl azide polymer, xenon hexafluoride, and ammonium perchlorate could be very hazardous to handle when finely divided, given their considerable surface area-to-volume ratio. The amount of energy needed to set off a decomposition or deflagration reaction would be reduced to hazardously low levels where such fine powders can be employed.
It was hypothesized that the surface energy of the particulate material could be controlled by addition of suitable surfactants such as sodium laurylsulfate, sucrose monolaurate, or dextrose to prevent agglomeration, which would de-stabilize the colloidal suspension. However, such surfactants would be unsuitable for use in highly oxidizing environments, since the surfactants, and in particular dextrose, might chemically react with the oxidizing substances.
Finally, phase changes of the suspending medium could lead to disruptions of the suspension while in storage. Such phase changes can include temperature cycling, freezing, and phase changes from saturated liquid/gas to supersaturated fluid. The effects of such phase changes can include vibration of the suspension and potential plugging of valves and piping.
The present invention solves the aforementioned problems associated with the related art as well as other problems by providing a propulsion system in which the advantages of stable oxidizers such as nitrous oxide and conventional fuels can be retained while increasing the specific impulse and density specific impulse of the system containing these propellants.
In accordance with an embodiment of the present invention, the inherent disadvantages of fluid oxidizers are overcome by the provision of a bipropellant system suitable for retaining both fluid-phase and solid-phase oxidizer propellants while avoiding the aforementioned problems associated with stratification of the two oxidizer phases. The fluid-phase and solid-phase oxidizers can be retained in their separated state, but under carefully controlled pressurized conditions, in a tank or vessel, such as an oxidizer storage chamber (also referred to herein as the first chamber).
The pressurized fluid in the oxidizer storage chamber is preferably retained under supercritical conditions, or as a compressed gas or saturated liquid. Sudden negative adjustments in pressure, which can be effected by opening the chamber to communicate with a combustion chamber (or a communicating means interconnecting the oxidizer storage and combustion chambers) having a pressure differential with respect to the oxidizer storage chamber, can result in sudden phase transitions or volumetric changes. When suddenly depressurized, a solid-phase particulate oxidizer rapidly mixes into and is suspended in the fluid phase oxidizer, thereby providing a mixed-phase oxidizer. As referred to herein, xe2x80x9cmixed-phasexe2x80x9d shall include a solid-liquid, solid-gaseous, or solid-liquid-gaseous mixture. The pressure drop is preferably so great that, at least after an initial discharged portion (which tends to contain a high concentration of particulates), the mixed-phase oxidizer is substantially homogeneously insofar as the fluid-phase oxidizer discharged from the storage tank contains a substantially uniform distribution of solid particles therein. As long as this mixing continues, and for a certain time thereafter, the solid oxidizer particulates remain in suspension, uniformly distributed throughout the fluid oxidizer in the oxidizer storage chamber. This phenomenon is referred to herein as a mixed-phase suspension process.
The present invention also provides a monopropellant fuel propulsion system in which a solid-phase fuel propellant or energetic propellant is retained with a fluid oxidizer under pressurized conditions, and a monopropellant fuel propulsion system in which a solid-phase oxidizer is retained with a fluid fuel under pressurized conditions.
The present invention further relates to a bipropellant fuel propulsion system, including a reverse-hybrid system, in which a solid-phase fuel propellant or energetic propellant is retained with a fluid-phase fuel under pressurized conditions.
In addition, the present invention also is directed to methods of propelling an object in accordance with the mixed-phase suspension process of the present invention.
These and other objects, features, and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the present invention.